1. Field of the Invention
a. The invention relates to the field of an apparatus and fabrication of MEMS, inertial measurement unit (IMU), three dimensional packaging, micromachined inertial sensor and its microfabrication.
2. Background
As the demand grows for miniaturization of sophisticated equipment, technological advances in microfabrication are constantly being developed. For instance, integrated circuits have enabled millions of transistors to be built on a single die capable of processing billions of instructions per second. More currently, MEMS and nanotechnology have given birth to a new era of fabrication. Due to the fast growth of both fields, interest for miniaturization is being expressed by an increasing number of applications. A large portion of current research is focused on development of transducers. Multiple sensors measuring different phenomena are generally desired to be very compact, simplifying integration. Moreover, many applications such as inertial measurement, ultrasonic devices, and optical sensing benefit from measurement along multiple axes. To address the needs of current and future applications, a novel approach of creating a chip-level system of micro machined sensors (or transducers) capable of detection (or transduction) along three independent axes has been developed.
One application for such a device is inertial measurement. Fields such as defense, space exploration, navigation, and personal entertainment are showing a rising interest for a complete inertial measurement unit (IMU) capable of measuring rotation and acceleration along three independent axes. Another application is multi-axis pressure sensing. A miniature three dimensional microphone could be used to monitor audio signals with the ability to precisely detect the source location of individual sounds, or to detect all sound in three dimensions. Ultrasonic devices could be built to transmit and receive sonar signals for underwater exploration or communication. Video surveillance purposes could benefit from a three dimensional camera that minimizes the number of cameras needed to fully observe the surroundings. For applications requiring very low-power consumption such as space exploration or remote navigation, a lightweight multi-axis energy scavenging device can be built to efficiently store energy induced from inertial motion. Though this is only a short list of possible applications for a compact three dimensional system of transducers and sensors, this manuscript will focus specifically on the example of a chip-level Inertial Measurement Unit (IMU).
Current methods to create a compact IMU typically fall into three general regimes. The most common approach is to use off-the-shelf sensors mounted onto PCBs and assembled into a three dimensional configuration. A conceptual version of this is shown in FIG. 1 where a three dimensional structure can measure rotation rate and accelerometer if provided with a sensor PCB module on each sidewall of a cubic sensor structure. With this method, separately packaged, single-axis sensors are typically used which are optimized for performance and rejection of cross-axis inputs.
Therefore the overall IMU benefits from better performance compared to using multi-axis sensors. Also, current technology supports this approach as PCB manufacturing and assembly is a very mature and well-known process. However, major drawbacks do exist. First, significant PCB-level assembly of individual components is necessary. For this reason, alignment errors will vary for each device, requiring extensive calibration of each individual unit before use. Additionally, the device is inherently not very compact. While units developed currently are small (on the order of 1-2 in3), there is no clear path for further miniaturization and reduction of power consumption.
An alternative method used to create a chip-level IMU, shown in FIG. 2, is to fabricate all sensors onto a single die. This allows the footprint of the system as a whole to be small enough for chip-level packaging. Lithographic alignment also reduces the difficulty of post-fabrication calibration because the sensors are all nearly orthogonal to each other. Despite these advantages, there are significant drawbacks to this IMU architecture. First, creating sensors for in-plane and out-of-plane detection requires very different design parameters. For both to coexist on the same substrate, a compromise in fabrication complexity and sensor performance must be made to accommodate multiple axes on the same substrates. It is often that the in-plane and out-of-plane sensors are created with mixed technologies, such as a combination of bulk and surface micromachining, sacrificing performance in some axes.
Another emerging method for creating micro IMUs involves chip-stacking as shown in FIG. 3. Each sensor is fabricated independently and then known-good dies are stacked together onto a single chip. In comparison to the PCB and common substrate approaches, the overall size of the IMU is reduced to a footprint equal to that of one sensor. Also, because each die is fabricated independently of the stack, signal detection electronics can also be included in the stack to create a self-contained chip-scale system of sensors. However, there are drawbacks to this approach as well. In most cases, both in-plane and out-of-plane sensors are included because of the stack geometry. Because the sensors are created with different processes, they are not identical in design and suffer from mismatched sensitivities between the in-plane and out-of-plane devices. Another challenge with this method is minimizing electrical crosstalk. Due to the large number of interconnects that are required to pass through the bottom chip, significant parasitic capacitance exists which induces noise and degrades IMU performance.